As pointed out by mozillalinks, Mozilla is planning to do a major overhaul of their add-on manager UI for Firefox. The new add-on manager will change from the existing popup window, to its own tab.

The add-on manager will move from the existing top navigation to side-tabs, keeping the same list view of available and installed add-ons. Users will be able to manage their languages, search engines, extensions, themes and plug-ins all in the same window, being able to search for addition add-ons through the built in Mozilla search.

The add-on manager now includes search engines, where previously only available on its own, apart from the add-on window. Mozilla has not announced their default list of search engines yet, but will allow a undo and restore search engine option.

Some of the functionality and UI isn't final yet, and is currently very buggy.